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tle oil comes with i t , i t is removed by filtering through dry filter paper. An aliquot part, depending on the salt content, is then concentrated for titration with a solution of approximately 0.05 N silver nitrate, using potassium chromate as a n indicator. From these results the chlorine can be calculated, or, if previous analysis of similar brine has shown i t t o consist mostly of sodium chloride, i t may be calculated as such. We have tested the accuracy of t h e method by making a re-treatment of some of the oil which had settled out after the treatment outlined above. A mere trace of chlorides could be found, thus furnishing good proof t h a t the first treatment had effected their almost complete removal.
Vol. 13, No. 4
The method has been used for about a year, and t h e following check results, expressed as grams of salt per liter of oil, have been obtained: Gravity of Crude O
Be.
B. S. & W.
Per cent 1.0 2.2 0.6 0.9 1.6 0.8
30.9 36.5 31.3 35.8 32.3 31.4
Water by Dist.
First Result
Per cent 0.8 1.6 0.5 0.8 1.1 0.6
0.77 0.49 0.72 0.35 0.60 0.67
G.
Second Result
G. 0.75 0.50 0.75 0.37 0.59 0.68
ACKNOWLEDGMENT
Experimental work on t h e method was carried out in this laboratory by Messrs. Philip A , Crosby and John G. Campbell.
LABORATORY AND PLANT Humidity Control by Means of Sulfuric Acid Solutions, with Critical Compilation of Vapor Pressure Data' By Robert E. Wilson RESEARCH LABORATORY OR APPLIBDCHEMISTRY, MASSACHUSETTS INSTITIJTE h7EED FOR HUMIDITY
CONTROL I N LABORATORY W O R K
I n t h e course of both research and routine laboratory work, many occasions arise when it is desired t o maintain a definite humidity in an enclosed space or t o produce a stream of air of definite moisture content. I n studying the humidity equilibria and rate of drying of various substances, such control is, of course, a prime requisite. There are, however, many other properties of materials which vary greatly with changes in their moisture content. I n order t o obtain reproducible results in any investigation which relies upon the quantitative measurement of such properties, it is therefore necessary either t o test t h e materials in an atmosphere of a definite humidity, or else, when the time of t h e test is short compared with t h e rate of taking up moisture, previously t o equilibrate them with a definite humidity. I n cases where only a single humidity is t o be used for such tests, this Laboratory has adopted 50 per cent relative humidity as a standard for articles which are t o be tested under conditions approximating those prevailing indoors, and 65 per cent humidity t o approximate those prevailing outdoors. I n many cases, however, it is necessary t o make t h e tests under a variety of conditions. Accurate control of temperature is generally not as important as control of humidity, as t h e moisture content of most materials varies but little with moderate changes in temperature, providing t h e relative (not absolute) humidity is kept constant. The object of this article is not t o suggest any new methods of obtaining this humidity control, but merely t o present in convenient form t h e d a t a which this laboratory has compiled from t h e literature, or found by practical experience, with reference t o what seems t o be t h e most satisfactory method of small-scale humidification, namely, t h e use of sulfuric acid solutions of definite composition. 1
Received December 14, 1920.
OF
TBCHNOLOGY, CAMBRIDGE, MASSACHUSETTS
ADVANTAGES
OF
S U L F U R I C ACID
SOLUTIONS
FOR
HUMIDITY C O N T R O L
Sulfuric acid solutions have many advantages over other materials which might conceivably be used for this purpose. Homogeneous solutions varying from 0 t o 100 per cent water can be obtained; t h e vapor pressure of these solutions has been much more accurately determined t h a n for any other concentrated solutions; the composition, a n d hence t h e vapor pressure, of t h e solutions can be quickly and accurately determined by measuring their density, which varies greatly with changes in composition; their relative vapor pressure (per cent of t h a t of pure water a t t h e same temperature) varies but little with wide changes in temperature; they come t o equilibrium rapidly with t h e surrounding atmosphere; the sulfuric acid itself exerts no appreciable vapor pressure; and finally, material of adequate purity is cheap and readily obtainable. For t h e purpose of maintaining a constant humidity in a closed chamber, sulfuric acid solutions have no real competitor under ordinary conditions, since it is merely necessary t o place within the chamber some acid of t h e proper strength with a n amount of surface exposed in general somewhat larger t h a n t h a t of any other moist or hygroscopic material present. When a fairly large stream of humidified air is t o be produced i t is, of course, possible t o obtain i t by mixing two streams of air, one of which has been thoroughly dried and one of which has been bubbled through water. By varying t h e relative amount of t h e two streams i t is possible t o obtain any desired humidity. This necessitates, however, both drying and humidifying fairly large amounts of air, and also the maintenance of an absolutely constant ratio between t h e two streams. It also requires frequent analytical control which, a t lower temperatures, necessitates t h e use of a n absorption method. At higher temperatures wet and dry bulb thermometers
Apr., 1921
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
may be employed t o determine t h e humidity, but this method necessitates t h e rejection of t h a t fraction of t h e air which passes over t h e wet bulb a n d thereby picks up a n uncertain amount of additional moisture. Under ordinary conditions in t h e laboratory this method is much more cumbersome and expensive t h a n t h e simple procedure of bubbling the stream through sulfuric acid of a definite composition. T h e former method does, however, find some application under certain exceptional conditions where fairly high humidities are t o be produced and there is available a n air supply of reasonably constant low humidity which does not require chemical desiccation. Since t h e sulfuric acid method has t h u s been shown t o be remarkably well adapted for all-round purposes as a laboratory method of humidification, this Laboratory has made a careful study and compilation of t h e best available d a t a on t h e vapor pressure and density of sulfuric acid solutions as dependent upon their composition a n d temperature. COMPILATION 0%' VAPOR P R E S S U R E DATA
The first step In compiling t h e vapor pressure d a t a was t o plot on a large scale t h e results of four investigatorsl who have determined t h e vapor pressure a t 25" C. of sulfuric acid solutions as a function of their concentration. These results showed a surprisingly good concordance throughout t h e middle range of concentrations (30 t o 60 per cent HzS04, or 17 t o 76 per cent relative humidity), but somewhat larger deviations a t t h e two extremes. It therefore appeared desirable t o make use of t h e very careful work of Dieterici,2 who unfortunately made his measurements a t 0 " C. It is possible, however, by a simple a n d surprisingly accurate thermodynamical calculation (made as described hereinafter) t o convert these results over t o t h e corresponding values for 2 5 " C. T h e addition of this series of points left little doubt as t o t h e precise location of t h e curve except in t h e range between 65 and 85 per cent HzS04. T h e uncertainty in this region might be expected on account of t h e very low vapor pressures exerted by such solutions a t 25' C. (0.2 t o 2.3 mm.), and especially a t 0 " C., which makes their accurate measurement extremely difficult At; higher temperatures, however, these vapor pressures become quite large and can be measured readily a n d accurately. Fortunately, three investigators have determined t h e vapor pressure of such concentrated solutions a t temperatures of 75" or 100" C., a n d their d a t a were also calculated t o 2 5 " C. by similar thermodynamical calculations. When these results were compared with those measured a t 25" C., i t appeared quite certain t h a t Sorel's low temperature results were somewhat too high in this range, while those of Regnault and Bronsted were more nearly correct. This is not surprising in view of t h e fact t h a t if Sorel's original d a t a in t h e low temperature-high concentration range are plotted, the points vary widely from a smooth curve, a n d do
.
1 Regnault, Ann. chim. phys., 131 15 (1845), 179; Sorel, Z . angcw. Chem., 1889, 272. Helmholtz, Wied. A n n . , 27 (1886), 532; Bronsted, 2. 9hysik. Chem.. 68 (19091,693. * Dieterici, Wicd. Ann., 60 (1893),60: 64 (1897),616.
327
not correspond with his own figures obtained a t higher temperatures, when they are calculated over by thermodynamical methods. It is t h u s possible t o draw a vapor pressure-composition curve for sulfuric acid solutions a t 2 5 " C. (see heavy line, Fig. 1) which is probably accurate within 0.1 mm. throughout. It will be noted t h a t relative vapor pressures (per cent of t h a t of pure water a t t h e same temperature, thus corresponding t o the relative humidity of t h e air) are plotted rather t h a n t h e absolute values in millimeters, because t h e former vary but little with temperature, a n d also because they are more significant for most laboratory purposes. The absolute vapor pressures a t any temperature can readily be calculated by reference t o steam tables or other sources of vapor pressure d a t a for pure water. The encircled points in Fig. 1 all correspond t o actual measurements a t 25" C., while those in triangles are points calculated t o 2 5 " from measurements made a t other temperatures. Table I presents t h e original observed values a n d those calculated over t o 25" C . for all t h e latter group of points. TABLEI-CALCULATED VALUESFOR RELATIVE VAPORPRESSUREOF DILUTE SULFURIC ACID AT 25' C. Calculated Relative Relative Vapor Vapor Vapor H&3Oa Temp. Pressure Pressure Pressure t C. Mm. a t t O C. at 25' C. INVESTIGATOR Per cent 0 98.0 98.1 Dieterici.. 5.62 4.535 0 96.4 96.6 Dieterici., 9.24 4.452 0 92.7 93.1 Dieterici. 15.73 4.284 0 20.8 4.065 87.9 88.6 Dieterici.. 27.2 80.3 Dieterici.. 0 3,664 79.3 69.3 71.2 Dieterici.. , 32.8 0 3.200 0 66.2 Dieterici.. 63.9 35.4 2.952 0 52.7 55.2 Dieterici.. 40.5 2.435 0 40.8 37.8 Dieterici.. , 47.3 1.748 26.1 0 29.1 Dieterici.. 53.4 1.206 12.3 0 61.3 14.7 Dieterici.. 0.569 11.6 15.9 Burt. 75 45.9 62.8 0 68.5 Dieterici.. 0.164 3.5 4.6 100 57.0 7.5 3.7 70.8 Burt. 12.1 75 4.2 2.3 74.0 Sorel. 20.2 Briggs . , 77.5 100 2.66 0.94 78.0 7.0 75 1.14 Sorel 2.4 100 79.2 14.3 Briggs. 1.88 0.61
... . ...... ... ... ...... .. ...... .. . . ....... .. ........ .. . . . . . ...... . .......... .. ..
Again using thermodynamical methods, i t is possible t o calculate, from t h e accurately located 25" C. curve, similar curves for other temperatures a t which little or no direct experimental d a t a are available. Such curves (dotted) are also shown in Fig. 1 for O " , 50°, a n d 75" C. By means of these curves it is possible t o determine readily a n d accurately t h e vapor pressure of a n y sulfuric acid solution a t a n y temperature between 0" and 100" C. I n order t o make possible t h e reproduction of these curves on a large-scale plot, a series of points are presented in Table 11. TABLE 11-BEST VALUESFROM VAPORPRESSURECURVESFOR SULFURIC ACID SOLUTIONS &SO4 Per cent 0 5 10 15 20
25 30 35 40 45 50 55 60 65 70 75 80
Relative Vapor Pressure Values a t 00 c. 25' C. 500 c. Per cent Per cent 100.0 100.0 98.5 98.5 96.1 96.3 92.9 93.4 88.5 89.3 82.9 84.0 75.6 77.2 66.8 68.9 56.8 59.3 46.8 49.5 36.8 39.9 26.8 30.0 17.2 20.0 9.8 12.0 5.2 6.7 2.3 3.2 0.8 1.2
Per cent 100.0 98.4 95.9 92.4 87.8 81.7 73.8 64.6 54.2 44.0 33.6 23.5 14.6 7.8 3.9 1.6 0.5
750 c. Per cent 100.0 98.6 96.5 93.8 90.0
85.0 78.6 70.8 61.6 52.0 42.8 33.0 22.8 14.2 8.3 4.4 1.8
It will be noted t h a t for a temperature change of 5" or 10" C. t h e relative vapor pressure of most of the
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Vol. 13, No. 4
P E m m STZ~UZIGAGilZ) FrG. 1
solutions is practically constant; furthermore, t h e small increase which does occur with increasing ternperature is in t h e right direction t o compensate for a similar slight tendency on t h e part of practically all materials whose humidity equilibria have been determined.l It is therefore not necessary when studyIt is planned in the near future to present values which have been determined by this laboratory and others for the humidity equilibria of 1
various substances, such as wood, paper, cotton, silk, wool, jute, leather, rubber, carbon black, etc.
ing such equilibria t o attempt t o maintain the system a t a temperature any more constant t h a n t h a t of t h e average laboratory. The information most frequently desired, however, is not what vapor pressure is exerted by a solution of given concentration, but what concentration of acid should be used t o obtain a given vapor pressure. This information is conveyed in Table 111, which is also determined from Fig. 1 drawn on a large scale. I
Apr., 1921
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
TABLE111-STRENGTH OF HzSOa REQUIRED TO GIVE DEFINITE HUMIDITIES --Per cent Sulfuric Acid Required atRelative Humidity Per cent 00 c. 25' C. 50' C . 7.50 10 63.1 64.8 66.6 68.3
c.
25 35 50 65 75 90
54.3 49.4 42.1 34.8 29.4 17.8
55.9 50.9 43.4 36.0 30.4 18.5
'
57.5 52.5 44.8 37.1 31.4 19.2
59.0 54.0 46.2 38.3 32.4 20.0
PRACTICAL U S E O F T H E VAPOR P R E S S U R E CHART
since density determinations afford a very satisfacto,:y and rapid method of determining the exact concentration of sulfuric acid solutions, Fig. 1 also includes curves showing t h e variations in this property with temperature and concentration. These are constructed from t h e very accurate d a t a of Domke.' T o prepare a solution having a specified vapor pressure a t a given temperature, i t is therefore only necessary t o refer t o Fig. 1 t o find t h e proper concentration of acid, and also t h e density of t h e solution. T h e temperature at which t h e density is determined need not be t h a t a t which t h e solution is t o be used, providing t h e proper curves are used. Thus t h e density curves in Fig. 1 indicate t h a t in order t o obtain a relative humidity of 50 per cent a t 2 5 " C., 43.4 per cent HzS04 should be used, and this acid has a density of 1.329 at t h e same temperature. If, however, it be desired t o determine t h e concentration before t h e solution has had time t o cool after pouring t h e strong acid into t h e water, another line shows t h a t the density of t h e acid at 50" C. is 1.311. It is unwise t o attempt t o measure densities a t temperatures higher t h a n this, but t h e values for intermediate temperatures can readily be determined by interpolation. Either vapor pressure or density values a t any temperature between 0 " and 100" C. can readily be obtained by a simple inter- or extrapolation. Sufficiently accurate density determinations can be made by a n y properly calibrated (water at 4' C. = 1) Westphal balance or hydrometer reading t o three decimal places, a pycnometer being more accurate t h a n is necessary. RECOMMEXDED
-METHODS
OF
DETERMINING
329
humidity in question. I n order t o make certain t h a t substantial equilibrium has been reached, in any given case, i t is always desirable t o approach i t from both t h e dry and t h e moist sides.' The simplest way t o accomplish this and t o determine all t h e points on t h e curve with a single sample is t o pass fairly dry air through a t t h e start, and then determine in t u r n the equilibrium weights at 10, 25, 50, 75, and 9 0 per cent humidity; saturated air is then passed through for a short time and t h e same points redetermined in t h e Y e V e Y S e order, finally obtaining dry weight a t the elevated temperature. Moisture contents should preferably be expressed as per cent Of the d r y weight* I n general, equilibrium can be reached from 18 t o 96 hrs., and t h e d r y weight within 2 t o G hrs., depending largely on t h e state of subdivision of t h e material. Higher water contents and lower temperatures require t h e longer times. These rates are much more rapid t h a n can be obtained by t h e frequently used method of exposing t h e sample over sulfuric acid, where t h e r a t e of approaching equilibrium has been found t o be surprisingly slow, due t o t h e slow rate of diffusion in still air. If a small fan be used in t h e desiccator, and a stirrer in t h e acid, t h e rate can b e made t o approach t h a t of t h e tube method, but this. is difficult t o arrange in an ordinary desiccator. T h e use of vacuum greatly increases t h e rate of approaching equilibrium, but is quite likely t o introduce errors o n account of t h e inrush of unconditioned air when t h e vacuum is broken preparatory t o removing and weighing the exposed sample.
HUMIDITY
E.QUILIBRIA
I n determining t h e humidity equilibrium of any substance a t a given temperature, t h e most satisfactory general method is t o subdivide it until t h e amount of surface exposed is reasonably large, place from 20 t o 60 g. in a small straight or U-tube of known weight,andpass a slow stream (50 to 500 cc. per min,, using t h e higher rates a t lower temperatures) of properly humidified air through it, rn no any glass wool or cotton be used in t h e tubes. The t u b e containing t h e material is weighed every few hours until constant weight is reached, after which t h e moisture content may be determined, preferably by passing through it a stream of warm air (500 to 125" C., depending on t h e nature of t h e material) previously dried by pz05, 'Onstant weight is reached. ' T h e loss in weight, of course, represents t h e equilibrium moisture contentat t h e temperature and 2. p h j s i k . Chpm., 43 ( 1 9 0 3 , 125; also Landolt-Bhmstein, 1912, 265.
FIG.2 - A P P A R A T U S
FOR
DSTERMINING HUMIDITY
EQUILIBRIA
A few precautions must be observed in conditioning t h e air stream. If t h e rate of flow is more t h a n about 100 cc. per min.2 Some special form of bubbler, designed t o give good contact between liquid and gas, should be employed. A petticoat type such as t h a t indicated in Fig. 2 has been found t o give excellent results. The use of broken glass or beads in the acid bottle also aids in distorting or breaking up t h e bubbles 1 While the two figures thus obtained will generally agree within narrow limits, some colloidal materials which tend to be highly hydrated or form gels will exhibit different apparent equilibrium values, depending on the side from which it is approached. This appears to be due to a hysteresis effect frequently observed in such materials, which may require a matter of months or years to reach substantially the same structure when approaching the same point from opposite sides.
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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
of air and bringing them to equilibrium. With such bubblers, substantial equilibrium can ordinarily be reached, even with only two bottles in series, a t rates of flow up t o 2 liters per min. I n case t h e entering air is very far from the desired humidity, or if the rate of flow is rather high (2 t o 10 liters per min.), three bottles should be used in series. If for any reason it is desired t o humidify larger amounts t h a n this, either two parallel lines of bubblers or a tower filled with glass beads, over which acid is trickling, should be used. The recommended size of the acid bottles varies from about 500 cc. for the slower rates of flow t o 2 liters for the higher. They should be filled about half full of acid. I n order t o determine when t h e acid needs t o be replaced, i t has been found desirable t o mark t h e initial level of acid in each humidity bottle. If three bottles are used, as is recommended for most purposes, the volume of acid in the first bottle can be allowed t o change by 3 or 4 per cent before i t is necessary t o add water or acid t o restore t h e original concentration. The density of t h e acid in the second and third bottles should be checked up occasionally, but will usually be found t o change but little if t h e concentration in t h e first bottle is properly adjusted. If the humidity of t h e entering air is known t o be considerably lower t h a n the desired value it is generally desirable t o have the initial water content of t h e $rst bottle 2 or 3 per cent higher t h a n t h e t r u e equilibrium concentration used in the last two bottles, and vice versa, if t h e entering air is too moist. This brings t h e air t o equilibrium more rapidly, a n d makes t h e first bottle serve just so much longer before i t goes too far t o t h e other side of the equilibrium concentration and must be renewed. For reasons previously pointed out, i t is ordinarily not necessary t o control t h e temperature, as variations of 5 ” or 10’ C. in t h e temperature have practically no effect upon humidity equilibria. One important precaution t o observe is t h e use of a tube rather tightly packed with glass wool, or similar material, t o remove entrained particles of sulfuric acid from t h e air stream lest they be deposited on t h e material t o be equilibrated. Quite appreciable amount$ may be carried over in this way unless this precaution is observed. Pig. 2 shows a typical set-up of apparatus for t h e rapid and accurate determination of t h e humidity equilibrium of a fibrous material such as cotton. METHOD O F CALCULATING T E M P E R A T U R E CORRECTIONS
Since i t might be desired to extend the foregoing d a t a over still wider temperature ranges, probably with some sacrifice of accuracy, i t appears desirable t o present as briefly as possible t h e method of calculating the vapor pressure values from one temperature t o another, together with t h e thermochemical d a t a on sulfuric acid solutions which was assembled for t h e purpose. The bas); of th- calculations was, of course, t h e approximate torm O L the Clausius-Clapeyron equation, ViZ.,
Vol. 13, No. 4
where A H is the heat absorbed in t h e evaporation of one mole of water from a large amount of t h e solution (so t h a t there is no .appreciable change in concentration). This is equal t o the sum of the heat effects involved in removing one mole of liquid water from t h e solution and in evaporating it, or t o a H 1 AH^; where AH^ is t h e molal heat of vaporization of pure water and a H z is t h e heat absorbed when one mole of water is removed from solution without change in concentration. Since the same equation applies t o pure water, vuiz.,
+
i t is possible t o eliminate t h e large quantity, aH1, by subtracting t h e second equation from t h e first, and t h u s obtain a very accurate expression for the ratio ps t o p w , which will be called r , t h e relative vapor pressure of the solution, as previously defined. This expression,
can be integrated on t h e assumption t h a t aHz is independent of t h e temperature,l giving the equation used in t h e calculations, namely,
To obtain t h e values of aHz for different concentrations, use was made of t h e d a t a of Bronsted2 on the heat of dilution of sulfuric acid, apparently a t 18’ C. These appear t o be better t h a n t h e earlier d a t a of Thomson, who probably had a small amount of water in his supposedly pure sulfuric acid. Bronsted gives values of Q (total heat of dilution) for t h e reaction 4-n HzO ----f HkSO4.n HzO Q, where 12 varies from 0 t o 15. I n order t o obtain similar values of Q for other temperatures it is necessary t o make use of Kopp’s Law,
+
where Z r l and 8 I ’ Z are t h e heat capacities of t h e reacting materials a n d t h e reaction products, respectively. I n comparing t h e results of various investigators on t h e heat capacities of sulfuric acid solutions, especially Thomson,8 B e r t h e l ~ t , ~Marignaq6 Cattaneo,B Pickering,’ a n d Schlesinger,* t h e agreement was not found t o be highly satisfactory. Fortunately, however, the specific heats are mere minor correction terms for t h e purpose in hand, and need not be known with great accuracy. T h e values used were taken 1 This involves no appreciable error since, as noted later, the value used for AHz was always that for the mean between the particular TIand Ta in question. a Z. physik. Chem., 68 (1909), 693. 8 Pogg. A n n . , [3] 30 (1853), 261; also “Thermochemische Untersuchung,” 3, 1. 4 A n n . chim. phys., [5] 4 (1875), 446. 5 Arch. 5’06. Phys., 39 (1870), 217; 55 (1876), 113. ,e Nuovo Cimenfo, [3] 26 (1889), 50. 7 J. Chem. Soc., 67 (1890), 91. * Physik Z.,10 (1909), 210.
T H E J O U R N A L OF I N D U S T R I A L A N D ENGIATEERING C H E M I S T R Y
A p r . , 1921
from a smooth curve which appeared t o be representative of t h e more recent a n d better work, and which may be reproduced from Table IV. TABLE IV-VALUES ASSUXED FOR
SPQCIFIC S O L U T I O N S A T 18'
Per cent HI SO^ 0 20 40 60
HEATS OB SULFVRIC ACID
c.
Specific Heat 1 .oo 0.84 0.68 0.53 0.41 0.34
80 100
331
are very low, so t h a t t h e error in calculating a n y vapor pressure from 25" t o 75' C. certainly cannot exceed 0.4 per cent relative vapor pressure. T h e maximum probable error from all sources of a n y of t h e curves shown in Fig. 1 is about 0.G per cent relative humidity. TABLEVI-VALUES Per cent HiSOa
OF
n
Hz
FOR
VARIOUSCONCENTRATIONS AND M E A N
TEMPERATURES
12.5'
500
37.50
62.5'
Using these data, t h e corresponding values of Q a t t h e desired mean temperatures were calculated and t h e results are presented in Table V. TABLE V-CALCULATED 1
n 1 1.5 2 3 4 7 9
15 19 a b
Per cent &SO4
HEATSO F DILUTION (Q) O F ADDINGli Hz0 AT DIFFERENT TEMPERATURES (In Calories per hiole of H z S O I )
TO
&So4
18'(obs.)
12.5O
37.5'
500
62.5'
84.5 78.4 73.1 64.5 57.7 43.8 37.7 26.6 22.3
TABLEVII-FACTORS TO 131.; APPLIEDTO R Q L A T I V Z V A P O R A T 25' T O GIVE RELATIVEV A P O R P R E S S U R E A T TEMPSRATURES INDICATED Per cent HIS04
00
20 30 40 50 60 70
0.992 0.978 0.956
80
500
1.007 1.020 1.043 1.084 1.156 I . 289
1.556
750 1.014 1.038 1.082 1.163 1.314 1.608 (2.295)
PRESSURS
1000
1.020 1.054 1.116 1.236 1.478 ( 1 .968) (3.240)
ACKNOWLEDGMENT
18,090 1.646
18,600 1.666
18,870 1.676
19,110 1.684
T h e amount of heat evolved (AH?) when one mole of water is added t o a large amount of a solution of a definite concentration is of course equal t o t h e rate a t which t h e total heat of dilution, Q, is changing a t t h a t particular concentration. This may be determined by plotting Q against 9% and determining by graphical methods t h e slope of t h e tangent AH^) a t a given concentration. I t was found more accurate and convenient, however, t o perform this operation analytically. Thornpsen had shown t h a t t h e heat of dilution of sulfuric acid could be expressed fairly accurately by a n equation of t h e form an Q = 9% b'
+
I n order t o determine t h e value of t h e coefficients a a n d b , i t is only necessary t o substitute two values of n and Q and solve t h e two equations simultaneously.1 Having obtained the equation of t h e curve, t h e values of AH* were readily and accurately calculated by taking t h e first derivative for t h e proper values of n, i. e.,
Tables V I and VII present values of AH^ a n d of : r1 ratios calculated in this way for various percentages of sulfuric acid. Using these ratios i t is obviously a simple matter t o calculate a n observed vapor pressure a t a n y one of t h e indicated temperatures over t o a n y other of these temperatures, or, by interpolation, t o intermediate temperatures. Although these correction ratios appear large in t h e case of t h e more concentrated solutions, i t should be remembered t h a t t h e vapor pressures in these regions y2
1 ,4ctually, in order t o distribute the inaccuracies over the curve, three pairs of values of n were taken, namely, 1 and 3, 1.5 and 4, and 2 and 7 , and the three values of a and b averaged. Using these average values of the coefficients (see Table V), it was found that the maximum deviation of the cdlctiloted from the original observed values of Q was less than 150 cal. up t o n = 4 and less than 300 from )t = 7 on.
The writer desires t o express his appreciation of t h e assistance rendered by Dr. D. R. Merrill in making t h e thermodynamic calculations involved in t h e preparation of this article. American Oil Chemists' Society For a number of years the leading chemists of the cottonseed oil industry have been associated in the Society of Cotton Products Analysts. As a result of the widening of the field of the vegetable oils which took place during the war, the Society was reorganized at its annual meeting last May, as the American Oil Chemists' Society. All persons engaged in chemical work on oils, fats, waxes, and allied interests are eligible for active membership, provided they have had a t least five years' chemical training. The Society publishes its transactions regularly and maintains the Chemists' Section in T h e Cotton Oil Press, devoted exclusively to the edible vegetable oil industry. The membership also includes those interested in the so-called technical aspects of the industry. Since most of the oils dealt with are industrially more or less interchangeable, their chemical control and technological development can be properly fostered only by intimate contact between science and industry. Any one interested in the activities of the new organization may obtain further information from the secretary, Mr. Thos. B. Caldwell, Wilmington, N. C.
T. A. P. P. I. Meeting The annual meeting of the Technical Association of the Pulp and Paper Industry is to be held at, the Waldorf-Astoria and the Hotel Astor, New York City, April 11 to 14, 1921. Engineering problems in the industry will be broadly considered in committee reports and in special papers. Among the subjects considered will be a new groundwood process, preliminary impregnation of wood as a means of shortening the cooking time in the sulfite process, the operation of water-power plants at maximum efficiency, the measurement of moisture in chips for cooking, the testing of crude rosin, methods of drying paper on paper machines, and the electrification of paper machinery. On Wednesday, April 13, a discussion, in charge of the committee on heat, light, and power, will be conducted on Pulverized Fuel and Steam Economy.
